WAEC Chemistry 2022 Paper

Concept:The gas liberated is a reducing gas, commonly sulphur dioxide, produced when a sulphite salt reacts with dilute hydrochloric acid.Explanation:Sulphite ions react with hydrogen ions from dilute hydrochloric acid to give sulphur dioxide:$\mathrm{SO}_3^{2-} + 2\mathrm{H}^+ \rightarrow \mathrm{SO}_2 + \mathrm{H}_2\mathrm{O}$Sulphur dioxide is a colourless gas with a pungent choking smell.It is a reducing agent and reduces acidified potassium manganate(VII), $\mathrm{KMnO}_4$.In acidic solution, the purple $\mathrm{MnO}_4^-$ ion is reduced to colourless $\mathrm{Mn}^{2+}$ ions.Therefore, the purple acidified $\mathrm{KMnO}_4$ solution loses its colour.It would not decolourise acidified $\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7$; that solution turns green instead. It also does not blacken lead nitrate paper.Answer:B. acidified $\mathrm{KMnO}_4$ solution colourless.